Units of heating, ventilating and air conditioning (HVAC) equipment, including variable capacity furnaces for heating enclosed spaces, such as residential dwellings and commercial buildings, have been developed in the interest of providing equipment which is more efficient and provides greater comfort for occupants of such spaces. Variable capacity furnaces, for example, typically include variable speed air circulating blowers and controls for providing heated air at different heat rates and air flow rates.
There are situations wherein the capacity requirements for heating or otherwise air conditioning an enclosed space require so-called twinning of furnaces and other air conditioning equipment, such as air handlers. Twinning typically involves equipment installations where separate multiple air conditioning units are connected to a common temperature controller or thermostat and are operable to discharge heated or cooled air into a common plenum or air supply duct for circulation to an enclosed space. Typically the return air from the enclosed space also flows through a common return air duct or plenum. For such twinning applications, certain types of air conditioning equipment cannot be used since operation of one unit at a specific capacity, for example, may differ from the operating conditions of the other unit or others of multiple units connected to the same ducting system. Accordingly, certain variable capacity units or units of different capacities may not be twinned or ganged since unequal heat output and pressures generated by different air flow rates of the respective units, may cause adverse operating conditions. Thus, it is important to be able to prevent twinning or mismatching of units of air conditioning equipment in applications where multiple units of such equipment have been specified. It is to these ends that the present invention has been developed.